Cell relay networks are commonly employed to transfer digital information over long distances. A typical cell relay network is comprised of a set of communication controllers coupled together for communication over common carrier communication links. A variety of communication devices may be coupled to the communication controllers over local communication links.
Such a cell relay network enables a variety of communication devices coupled to the communication controllers to share the common carrier communication links. The communication devices transfer digital information in the form of communication cells or packets over the common carrier communication links on a demand driven basis. The demand driven sharing of the common carrier communication links in such a cell relay network reduces the cost of maintaining a long distance communication network.
Typically, each communication cell transferred over such a cell relay network contains a cell header that specifies a destination address. The destination address identifies a virtual circuit connection on the cell relay network. The cell header specifies a virtual circuit (or a group of virtual circuits) which interconnect a source and destination pair.
For example, a prior cell network interface standard provides a 32 bit destination address format. The destination address comprises a virtual path identifier field (VPI), a virtual circuit identifier field (VCI), and a standard specific control field. The destination address optionally comprises a generic flow control field. The VPI is commonly used to group communication cell transmission through the cell relay network for a set of communication devices. The VCI is commonly used to uniquely identify communication devices within the VPI groups.
A communication controller in such a cell network typically decodes the destination address of an inbound communication cell to determine whether the communication cell is a pass through communication cell or a locally terminated communication cell. A locally terminated communication cell contains a destination address that specifies a local circuit connection to the communication controller. A pass through communication cell contains a destination address that specifies a circuit connection to some other communication controller in the cell relay network.
For example, such a communication controller may decode the VPI and VCI portions of the destination address to determine whether an inbound communication cell is locally terminated. The communication controller may decode the VPI portion of the destination address to route a pass through communication cell to the appropriate destination.
Typical prior communication controllers implement a look-up table mechanism to decode the destination address of inbound communication cells. However, a look-up table that fully decodes the VPI and VCI fields of an inbound communication cell requires an extremely large look-up table. Such a large look-up table greatly increases the cost of the communication controller, and reduces the speed of communication cell processing.
As a consequence, prior communication controllers usually simplify the look-up table mechanism by limiting the number of useful destination address bits. For example, one prior communication controller limits the VCI field to 10 valid bits and the VPI to 5 valid bits. Such limitations simplify the look-up function, and reduce the amount of memory required to implement the look-up table.
Unfortunately, such prior communication controllers having a limited number of useful destination address bits severely limit the flexibility of cell relay network configuration. Such prior communication controllers limit the number of available destination addresses, and thereby impose limits on the assignment of destination addresses to communication devices. Such prior communication controllers also impose limits on the grouping of cell transmissions by limiting the usefulness of virtual path identifiers.